This invention relates to scatter type smoke detectors. Detectors of this type include an exciter lamp illuminating a chamber in which particulate matter such as smoke is to be detected. A smoke sensing photocell is coupled to the chamber and constantly senses a very low illumination emanating from the exciter lamp. When particulate matter enters the smoke chamber, light is scattered to the cell's photosensitive surface and the cell's light response increases triggering a threshold circuit to operate an alarm.
False alarms may be indicated by the smoke detector in the event excessive ambient light is permitted to enter the smoke chamber or the light emanating from the exciter lamp is not kept at a sufficiently low level.
Accordingly, seemly inconsistent criteria must be satisfied in keeping ambient light, as well as light from the exciter lamp, at a relatively low level; and at the same time, enabling particulate matter, such as smoke, easy access into and out of the smoke detector.